In general the function of a circuit breaker is to electrically engage and disengage a selected circuit from an electrical power supply. This function occurs by engaging and disengaging a pair of operating contacts for each phase of the circuit breaker. The circuit breaker provides protection against persistent overcurrent conditions and against the very high currents produced by short circuits. Typically, one of each pair of the operating contacts are supported by a pivoting contact arm while the other operating contact is substantially stationary. The contact arm is pivoted by an operating mechanism such that the movable contact supported by the contact arm can be engaged and disengaged from the stationary contact.
There are two modes by which the operating mechanism for the circuit breaker can disengage the operating contacts: the circuit breaker operating handle can be used to activate the operating mechanism; or a tripping mechanism, responsive to unacceptable levels of current carried by the circuit breaker, can be used to activate the operating mechanism. For many circuit breakers, the operating handle is coupled to the operating mechanism such that when the tripping mechanism activates the operating mechanism to separate the contacts, the operating handle moves to a fault or tripped position.
To engage the operating contacts of the circuit breaker, the circuit breaker operating handle is used to activate the operating mechanism such that the movable contact(s) engage the stationary contact(s). A motor coupled to the circuit breaker operating handle can also be used to engage or disengage the operating contacts. The motor can be remotely operated.
A typical industrial circuit breaker will have a continuous current rating ranging from as low as 15 amps to as high as several thousand amps. The tripping mechanism for the breaker usually consists of a thermal overload release and a magnetic short circuit release. The thermal overload release operates by means of a bimetalic element, in which current flowing through the conducting path of a circuit breaker generates heat in the bi-metal element, which causes the bi-metal to deflect and trip the breaker. The heat generated in the bi-metal is a function of the amount of current flowing through the bi-metal as well as the period of time that that current is flowing. For a given range of current ratings, the bi-metal cross- section and related elements are specifically selected for such current range resulting in a number of different current ranges for each circuit breaker. Electronic trip units are also used in some applications.
In the event of current levels above the normal operating level of the thermal overload release, it is desirable to trip the breaker without any intentional delay, as in the case of a short circuit in the protected circuit, therefore, an electromagnetic trip element is generally used. In a short circuit condition, the higher amount of current flowing through the circuit breaker activates a magnetic release which trips the breaker in a much faster time than occurs with the bi-metal heating. It is desirable to tune the magnetic trip elements so that the magnetic trip unit trips at lower short circuit currents at a lower continuous current rating and trips at a higher short circuit current at a higher continuous current rating. This matches the current tripping performance of the breaker with the typical equipment present downstream of the breaker on the load side of the circuit breaker. Again, electronic trip units can also be used. Because of the higher voltages and currents that must be interrupted, there is potential for damage to the components of a circuit breaker from the hot by-products of the electric arc interruption. During an electrical interruption, both gasses and small molten metallic particles are generated and expand outward from the electrical contacts into the arc chamber area of the circuit breaker. One component of a circuit breaker that is particularly vulnerable to damage from arc exhaust is the contact pressure spring which biases the movable contact arm in its closed, "ON," position. If the spring is exposed to too much heat, this may cause annealing and the spring can lose tension. This could result in the spring's inability to close the contact arm after a fault current event.
Another problem occurs in circuit breakers subject to high continuous current ratings. In a circuit breaker that is subject to high current, the overall size of the breaker must be larger in order to accommodate conductors with a larger cross section. This means that the crossbar must be longer. In addition, because greater pressure is required to maintain the contacts, the movable contact and the stationary contact, in a closed position a greater force is transmitted to the crossbar. Because of the longer length and the greater forces on the crossbar, the crossbar has a tendency to flex or bow along its length when the circuit breaker is "ON" and the contacts are closed. In such situations, the crossbar flexes but the contact arm pivot remains stationary. As a result, the geometric relationship between the surfaces of the crossbar and the contact arm change which changes the amount of torque applied to the contact arm by the crossbar during normal operation or in a overload condition. Therefore, flexing of the crossbar can cause an unacceptable amount of variation in the pressure that must be applied to the contact arms to maintain the proper mechanical and electrical coupling with the contacts.
Thus, there is a need for a molded case circuit breaker that will protect a contact arm pressure spring from arc gases and debris and that works throughout a broad range of current readings with a minimum of unique parts and manufacturing tools. Further there is a need for a molded case circuit breaker that minimizes or eliminates the geometric changes between the crossbar and the contact arm pivot. There is an additional need for a molded case circuit breaker in which the force needed to reset the breaker does not have to overcome the spring force that maintains the movable contact arms in the "ON" position. There is a further need for a molded case circuit breaker that can be easily reconfigured over a broad range of current ratings by utilizing interchangeable parts and additional parts with a minimum of unique parts.